1. Field of the Invention
The present invention relates to a vibratory sensor and method.
2. Statement of the Problem
Vibratory sensors, such as vibratory densitometers and vibratory viscometers, typically operate by detecting motion of a vibrating element that vibrates in the presence of a fluid material to be measured. Properties associated with the fluid material, such as density, viscosity, temperature, and the like, can be determined by processing measurement signals received from motion transducers associated with the vibrating element. The vibration modes of the vibrating element system generally are affected by the combined mass, stiffness and damping characteristics of the vibrating element and the fluid material.
FIG. 1 shows a prior art vibratory sensor wherein a pickoff sensor generates an electronic vibration signal corresponding to a sensed vibration. The vibration signal will be substantially sinusoidal in nature. The vibration signal is received in a signal processor that measures or characterizes the vibration signal. The signal processor may comprise a coder-decoder (i.e., codec) in some embodiments. The signal processor determines both the frequency and the amplitude of the vibration signal. The frequency and amplitude of the vibration signal can be further processed to determine a density of an associated fluid, or can be processed to determined additional or other fluid characteristics.
In addition, the signal processor may generate a drive signal for the driver. The signal processor may process the vibration signal to create the drive signal. The drive signal may have a predetermined drive amplitude, wherein the predetermined drive amplitude may be fixed or varying. The drive signal may have a drive frequency that is based on the frequency and amplitude of the received vibration signal. The drive frequency may comprise a resonant frequency of the vibration sensor vibrating in a fluid to be characterized, wherein the resonant frequency is determined by adjusting the drive frequency until the amplitude of the received vibration signal is substantially at a maximum.
However, the signal processor may have a limited dynamic range and may not be capable of accurately and reliably measuring a frequency of an input signal that has a relatively small amplitude. The vibration signal has dynamic amplitude range that is significantly greater than the usable dynamic range of a typical stereo codec. In the case of a fork density meter, the dynamic range between a minimum pickoff amplitude to a maximum pickoff amplitude may be greater than 10,000, while a typical stereo codec may have a usable dynamic range of roughly 100 in order to maintain a stable closed loop drive with the frequency accuracy requirements of the system.
The prior art vibratory sensor may include a gain stage between the pickoff sensor and the signal processor. The gain implemented by the gain stage may be selected so that the vibration signal is amplified to substantially match the dynamic range of the signal processor. The frequency of the vibration signal may then be more easily and accurately measured.
The prior art vibratory sensor of FIG. 1 has drawbacks. Amplification of the vibration signal by the gain stage may enhance the ability of the signal processor to match the vibration signal to the signal processor's input and quantify the vibration signal frequency, but unfortunately this negatively affects the ability of the signal processor to quantify the vibration signal amplitude.
FIG. 2 shows a prior art vibratory sensor wherein the gain stage comprises multiple gain elements K1-KN, wherein switches S1-SN select a gain element for amplifying the vibration signal. This prior art circuit allows the vibration signal to be amplified by more than a single gain factor.
The process of switching the gains has drawbacks, however. One problem is that the gains must be scanned dynamically during initial operation (and during slug operation) to ensure that the system remains stable at all times. Another problem is that the act of switching from one gain to the next creates nonlinearity in the measurement of both amplitude and frequency, which can result in errors in the fluid measurements generated by the prior art vibratory sensor. Yet another problem is that each gain stage has a different phase relationship from the pickoffs, which must be compensated for. In order to maintain this phase relationship, separate compensation numbers must be calculated for each usable gain stage to ensure the sensor is always operating on the predicted measurement points, as any errors in the linearization in phase from one gain to another will result in another source of measurement error.